Olivine in komatiite records origin and travel from the deep upper mantle

The deep upper mantle is the main source of high-temperature magmatism, but the only known naturally occurring samples of high-pressure mantle constituents are mineral inclusions in diamonds. Trace elements in olivine crystals from the 3.33 Ga Commondale Greenstone Belt in South Africa reveal that these crystals formed in the deep upper mantle as high-pressure phenocrysts, and some perhaps even formed in the mantle transition zone (410–600 km) where they began as wadsleyite. The crystals were entrained within ascending komatiite magma and conveyed to the surface. The olivine crystals have the highest contents of Al2O3 (0.3 wt%) recorded in any terrestrial olivine, which is indicative of formation at high pressure. The deep mantle gave rise to Archean komatiites, extraordinarily hot magmas (up to 1700 °C), which provide insight into Earth’s early mantle evolution and the formation of most ancient continental and oceanic crust. In spite of extensive research since their discovery over 50 years ago, the origins of komatiites have remained contentious. Plumes—thermochemical instabilities originating at the core-mantle boundary—are the most likely source, but no direct evidence of a deep mantle origin of komatiite has yet been recognized.

Pyrrhotite–silicate melt partitioning of rhenium and the deep rhenium cycle in subduction zone

The Re-Os isotopic system serves as an important tracer of recycled crust in Earth’s deep mantle because of the large Re/Os ratios and time-integrated enrichment of radiogenic Os in Earth’s crust. However, the Re distribution in Earth’s known reservoirs is mass imbalanced, and the behavior of Re during subduction remains little understood. We performed laboratory experiments to determine the partition coefficients of Re between pyrrhotite and silicate melt (DRepo/sm) at 950–1080 °C, 1–3 GPa, and oxygen fugacities (in log units relative to the fayalite-magnetite-quartz [FMQ] buffer) of FMQ –1.3 to FMQ +2. The obtained DRepo/sm values are 200–25,000, which increase with decreasing oxygen fugacity and the total iron content (FeOtot) of silicate melt but decrease with increasing temperature or decreasing pressure. Applying DRepo/sm to constrain the behavior of Re during slab melting demonstrates that slab melts contribute minimal Re to the sub-arc mantle, with most Re dissolved in sulfides subducted into Earth’s deep mantle. Deep storage of recycled oceanic basalts and sediments can explain the mass imbalance of Re in Earth’s primitive mantle, depleted mantle, and crust.

Diamond and Other Exotic Mineral-Bearing Ophiolites on the Globe: A Key to Understand the Discovery of New Minerals and Formation of Ophiolitic Podiform Chromitite

Ophiolite-hosted diamond from peridotites and podiform chromitites significantly differs from those of kimberlitic diamond and ultra-high pressure (UHP) metamorphic diamond in terms of occurrence, mineral inclusion, as well as carbon and nitrogen isotopic composition. In this review, we briefly summarize the global distribution of twenty-five diamond-bearing ophiolites in different suture zones and outline the bulk-rock compositions, mineral and particular Re-Os isotopic systematics of these ophiolitic chromitites and host peridotites. These data indicate that the subcontinental lithospheric mantle is likely involved in the formation of podiform chromitite. We also provide an overview of the UHP textures and unusual mineral assemblages, including diamonds, other UHP minerals (e.g., moissanite, coesite) and crustal minerals, which robustly offer evidence of crustal recycling in the deep mantle along the suprasubduction zone (SSZ) and then being transported to shallow mantle depths by asthenospheric mantle upwelling in mid-ocean-ridge and SSZ settings. A systematic comparison between four main genetic models provides insights into our understanding of the origin of ophiolite-hosted diamond and the formation of podiform chromitite. Diamond-bearing peridotites and chromitites in ophiolites are important objects to discover new minerals from the deep earth and provide clues on the chemical composition and the physical condition of the deep mantle.

Comparative geodynamics and geothermy of the Alpine and Pacific belts. Mechanical-mathematical modeling

Формирование и эволюция геологических структур отражают взаимодействие коры и мантии. Актуальность работы определяется предметом исследования – решением задачи механико-математического моделирования формирования и эволюции геологических структур над поднимающимся мантийным диапиром. Для моделирования геологических процессов и эволюции геологических структур в связи с движениями глубинных слоев мантии были собраны и проанализированы все возможные геолого-геофизические данные и использованы механико-математические модели различной реологии. Геолого-геофизические данные для Альборанского, Балеарского, Тирренского, Эгейского, Ионического, Черного, Каспийского морей, Левантийской, Прикаспийской, Паннонской, Алеутской впадин, Охотского, Японского, Филиппинского морей собраны и проанализированы. Взаимодействие литосферы и астеносферы находит свое отражение в формировании и эволюции геологических структур. Зоны столкновения литосферных плит характеризуются высокими P-T условиями, высокой сейсмичностью, землетрясениями, вулканизмом, магматизмом и активными проявлениями геотермальной энергии: вулканами, минеральными водами, дегазацией, горячими источниками. Целью исследования является разработка адекватной модели формирования и эволюции геологических структур на поверхности Земли в связи с глубинными геодинамическими процессами. Методы работы. Для изучения динамики литосферы в процессе эволюции на больших временах использовались механико-математические модели геологической среды на основе модели многослойной высоковязкой несжимаемой жидкости. Для приближенного решения уравнений Навье-Стокса и уравнения неразрывности использовались метод разложения по малому параметру, метод последовательных приближений и метод сращиваемых асимптотических разложений. Моделирование дает возможность рассчитывать распределение P-T параметров в слоях осадочного чехла, коры и верхней мантии в процессе эволюции структур. Существование зон растяжения в задуговых бассейнах можно объяснить подъемом мантийных диапиров в результате геотермального эффекта и подъемом астеносферы в процессе столкновения глубинных мантийных потоков. Результаты работы. Результаты механико-математического моделирования показывают, что в процессе развития осадочных бассейнов над поднимающимся мантийным диапиром структура поверхностного свода сменяется структурой глубокой депрессии. В аналитическом решении найдены критические параметры задачи, связывающие форму диапира, его глубину и скорость подъема со структурой земной поверхности. Результаты моделирования исследованы на примерах геологического строения Альпийского и Тихоокеанского поясов и хорошо согласуются с геолого-геофизическими данными. The origin and evolution of geological structures reflect crust-mantle interaction. The relevance of the work is determined by the subject of the study - the solution of the problem of mechanical and mathematical modeling of the formation and evolution of geological structures above the rising mantle diapir. For simulation of geological processes and geological structures evolution in connection with deep mantle movements all possible geological-geophysical data were combined and analyzed and the mechanical-mathematical models of different rheology were used. Geological-geophysical data for Alboran sea, Balearic sea, Tyrrhenian sea, Aegean sea, Ionian sea, Levant sea, Black sea, Caspian sea, Pre-Caspian depression, Pannonian depression, Aleutian depression, Okhotsk sea, Sea of Japan, Philippines sea are combined and analyzed. Lithosphere-asthenosphere interaction is reflected in geological structures formation and evolution. The zones of the lithosphere plates collision are characterized by high P-T conditions, high seismicity, earthquakes, volcanism, magmatism and active geothermal energy manifestations: volcanoes, mineral waters, degazation, hot springs. The aim of the study is to develop an adequate model of the formation and evolution of geological structures on the Earth's surface in connection with deep geodynamic processes. Methods. To study the dynamics of the lithosphere in the process of evolution at long times, we used mechanical and mathematical models of the geological medium based on the model of a multilayer high-viscosity fluid. For the approximate solution of the Navier-Stokes equations and the continuity equation, the method of decomposition in a small parameter, the method of successive approximations and the method of splicing asymptotic decomposition were used. Modeling gives possibility to calculate P-T parameters distribution in the layers of sedimentary cover, crust and upper mantle in the process of the structures evolution. The existing of stretching zones in back-arc basins can be explained by upwelling of mantle diapirs as a result of geothermal effect and raising of asthenosphere in the process of collision of deep mantle flows. Results. The results of mechanical and mathematical modeling show that during the development of sedimentary basins above the rising mantle diapir, the structure of the surface vault is replaced by the structure of a deep depression. In the analytical solution, the critical parameters of the problem are found that relate the shape of the diapir, its depth, and the ascent rate with the structure of the Earth's surface. The results of modeling are investigated on the examples of the Alpine and Pacific belts geological structures and give good agreement with geological-geophysical data